It is necessary to remove field heat and to otherwise cool produce to preserve the produce for shipment to market. Some methods of cooling produce appear in U.S. Pat. No. 5,375,431, entitled Produce Cooler and Method of Cooling Produce, issued Dec. 27, 1994; in U.S. Pat. No. 5,386,703, entitled Apparatus and Methods for Vacuum Cooling Fresh Produce, issued Feb. 7, 1995; U.S. Pat. No. 4,576,014, entitled Produce Vacuum Cooler With Improved Venting, issued Mar. 18, 1986, and in U.S. Pat. No. 3,844,132 entitled Produce Cooler and Method of Cooling Product, issued Oct. 29, 1974. The text and drawings of these four patents are incorporated herein by reference as though fully set forth here. Some of these patents describe apparatus comprising a vacuum chamber for receiving produce, a vacuum pump, a refrigeration system for collecting evaporated water and a pump for spraying water onto the produce. The vacuum pump reduces pressure within the chamber to sub atmospheric level, causing evaporation of moisture from the produce. This evaporation removes heat from the produce, reducing its temperature. Water vapor formed by such evaporation condenses on cooling coils positioned above the produce. These refrigerated coils preferably condense and collect as much water as is feasible to prevent the water from reaching the vacuum pump. This water is collected and directed to a reservoir below the produce. The collected water, in preferred embodiments, is at a temperature in the range of about 33 to about 35 degrees Fahrenheit. Additionally, a water recirculating system can utilize water from the reservoir at the bottom of the vacuum chamber and spray it over the produce for further cooling effect. This reservoir water may be passed through a filtration device utilizing non-residual free radical chemical methods of filtration or ultraviolet light to reduce the micro biotic load and insure freshness.
The advent and increased popularity of processed lettuce, i.e, chopped or shredded lettuce enclosed in bags with special atmospheres, has generated a need for improved handling, pre-cooling and processing, and a need to minimize abused, desiccated or decaying lettuce.
At present, Iceberg lettuce is cooled without spraying water on to it. Romaine, Red Leaf, Green Leaf and other thinner leaf lettuces need sprayed water for adequate cooling. The water weight percentage range for Iceberg Lettuce is between 95.72 and 94.76 percent while the range for Romaine and the Leaf Lettuces is between 95.07 and 93.67 percent with most of this water stored in the stalk of the leaf. It has been determined that there is a one percent (1%) by weight moisture loss from produce tissue when vacuum cooling without water for every 10 degree F temperature reduction (Page 5 of Exhibit 1, USDA Market Research Report No. 469, 1961). This water loss through vaporization is pulled from the produce tissue. This water loss through evaporation amounts to 28.75 gallons of water evaporated for every 10 degree F temperature reduction for a typical twelve ton, 24,000-pound produce load.
______________________________________ 480 boxes/load .times. 50 pounds/box 24,000 pounds/load .times. .01 moisture loss 240 pounds water .times. .1198 pounds/gallons of water 28.75 gallons/10.degree. F. change ______________________________________
Introducing a water spray to the vacuum cooling process, as disclosed in U.S. Pat. No. 3,844,132, adequately cooled leafy lettuces such as Romaine, Red Leaf and Green Leaf that lacked enough water in their tissues to enable proper cooling. When a water recirculating spray system is used, the produce weight loss is reduced because water evaporates from the surface of the produce rather than being drawn from its tissues. When water is drawn from the tissues of produce, resulting desiccated leaves are unappealing to buyers and useless to lettuce processors. Originally (Exhibit 1, page 7), moisture loss was assumed to be uniform throughout a lettuce head because of the nature of vacuum pressure. However, moisture loss has been found to be concentrated in the outer leaves of the lettuce head. The moisture loss concentration is due to two factors: first, the inner leaves of Iceberg lettuce are coated with naturally occurring water; second, the outer leaves are almost always warmer (having absorbed sunlight and the rising ambient air temperature) and need more evaporation to achieve the desired cooling. The typical 3-4 percent moisture loss (change in temperature is about 30 to about 40 degrees) results in a 25-35 percent tissue loss (excluding core which is 5-11% additional loss) by weight to the package lettuce processors. However, even a one percent moisture loss concentrated in the outer, more desired green leaves of a head of Iceberg Lettuce results in a substantial loss of marketable tissue.
Flowing a large amount of cold water over produce and cooling it by simple heat transfer was thought to be beneficial. However, at 970.3 Btu's/pound of water evaporated, (1072 Btu at 4.6 mm pressure) the change of phase of water has proved to be a more efficient method of cooling. Also, leaving water on produce tissue has proved to accelerate produce decay. Water that condenses on the refrigerated coils may have a temperature in the range of about 33 to about 35 degrees Fahrenheit. At these temperatures, it is difficult to reduce and control the sub atmospheric pressure at a level sufficient to evaporate such water without the evaporation freezing the produce. Produce picked early in the morning with low (33-40.degree. F. ) tissue temperatures is especially vulnerable to vacuum infiltration and/or freezing damage because only a portion of the sprayed water is evaporated before the produce tissue temperature reaches 32 degrees F. Initial attempts at reducing the tissue loss in Iceberg Lettuce by using water during the vacuum cooling process failed because the low temperature of the reservoir spray water and the porosity of Iceberg tissue resulted in water being left in and on the tissue, leading to accelerated decay. This failure is highlighted by the fact that the bagged lettuce processing industry now primarily uses two separate procedures to dry (the centrifuge) and cool (forced air cooling) its processed lettuce. This combination of procedures is utilized despite the fact that there are a number of product quality and operational problems. The centrifuge baskets need to be limited in size to effectively dry the tissue and they cut the tissue of the produce being dried, especially the thin leaf and baby lettuces, resulting in a further reduction of usable processed tissue. Also, with the cooling of the lettuce done by transporting the tissue through a stream of cold air all of the employees in the processing area need to use winter clothing in order to protect themselves from the chill created by the cool air.
Therefore, a major problem facing produce processors using vacuum cooling with water today, is how much water to spray at what water temperature, and when to spray it to minimize tissue desiccation without leaving free water on the tissue. In making this determination, it is important to know the field temperature of the produce to be cooled, and the desired water temperature, so that the water on the surface of the produce can be evaporated entirely to achieve the desired cooling and desired moisture content. Lettuce, celery and the like that are picked in the morning may have less heat to be removed than that which is picked in the warm afternoon. Finished cooling temperature of produce should always be below 40.degree. F.; the ideal temperature range is between 33-38.degree. F.
Determining the average temperature of a number of boxes of produce to adjust the cooling process accordingly has been too cumbersome and time consuming a procedure. At this time, an operator of a vacuum chamber with or without water may insert a temperature reading probe into selected produce to determine its temperature and use this data to estimate the total heat to be removed. The operator can thereafter adjust a timer to start the flow of cooling water onto the produce after a predetermined sub atmospheric pressure is reached within the chamber. Another timer is set to terminate the process after an appropriate sub atmospheric pressure has been reached. The operator may lengthen the cooling time under sub atmospheric pressure if ambient air and produce temperatures rise.
An accurate method for assessing total heat within a vacuum chamber during a cooling process is to observe the sub atmospheric pressures at which evaporations (flashes) occur. The accuracy of this method rests in the thermodynamic properties of steam as set forth in the reference book with the same title by Joseph H. Keenan and Frederick C. Keyes, Exhibit 2. This text with its data for the liquid and solid phases section is the foundation for the physical properties of water. Disclosed in this text is that while water evaporates at 212.degree. F. at atmospheric pressure of 14.7 lbs/in.sup.2 if the pressure is reduced to 0.08854 lbs/in.sup.2 (4.6 mm) liquid water will change to vapor at 32.0.degree. F. Thus, a pound of water at atmospheric pressure occupies 27 cubic feet while at 0.08854 lbs/in.sup.2 a pound of water occupies 3306 cubic feet. (Exhibit 2, Table 1; Exhibit 1, Page 12, FIG. 5). These evaporations (flashes) take place when the sub atmospheric pressure within the chamber reaches the flash point of the water in and on the produce tissue (Exhibit 2, Table 1). After measuring the pressure at which evaporations/flashes that occur, an operator can determine the aggregate heat load in the chamber and can adjust the amount of water to be sprayed and amount of time needed for cooling.
It is an object of this invention to provide processes and apparatus to chill produce by removing surface water and a desired amount of interstitial water from the produce while minimizing desiccation in which the heat of a load of produce can be measured accurately, and the controls of the vacuum cooling equipment, water spray and water temperature adjusted accordingly.